Low temperature properties such as superconductivity are now widely used in a range of different applications including Magnetic Resonance Imaging (MRI), superconducting magnets, sensors and in fundamental research. Historically, the evaporation of cryogenic liquids such as nitrogen or helium has been used as a cooling mechanism in order to reach the low temperatures required for such applications. Cryogenic liquids, particularly helium, have associated disadvantages in that they are often “consumable” due to incomplete recovery of boiled off gas. Furthermore, such apparatus for storing or otherwise handling cryogenic liquids is often bulky and requires special handling procedures. Such apparatus and procedures are somewhat incompatible with patient care environments.
More recently, closed cycle refrigerators (CCR) have been used to replace cryogenic liquids in providing an alternative refrigeration mechanism. In contrast with the evaporation of cryogenic liquids, CCRs do not rely upon a phase change within the coolant. Instead, CCRs operate upon a principle of using the cooling which is associated with the work of compression and expansion of a working gas coolant. The term “mechanical refrigerators” is used herein to describe such apparatus although those of ordinary skill in the art will appreciate that the term “cryocooler” is synonymous with this term. This invention is directed primarily to pulse tube refrigerators (PTRs), although it can also be used in connection with other mechanical refrigerators such as Stirling cryocoolers and Gifford-McMahon coolers, amongst others.
PTRs use a working gas such as helium to provide cooling at relatively modest cooling powers, to temperatures below 4 Kelvin. These low temperatures are produced by expanding and compressing the working gas in a thermodynamic cycle. In order to run the cycle, a typical PTR system comprises three major components—a compressor, a valve assembly and a pedestal part. The compressor supplies the cryocooler with high pressure compressed gas such as helium via a high pressure line, and receives gas back from the cryocooler in a low pressure line. The pedestal part comprises pulse tube(s), heat exchanger(s) where the cooling power is supplied, and different regenerator materials for heat exchange with the incoming and outgoing gas.
The valve assembly connects the high and low pressure sides of the compressor to the pulse tubes and regenerators within the pedestal part, and controls the timing and distribution of gas flows between the compressor and pedestal part in order to effect the thermodynamic cycle and subsequent cooling.
PTRs are extremely advantageous since they are closed systems with few moving parts and are essentially lossless with regard to the working gas. For these reasons, they are attractive both technologically and commercially, and the use of PTR systems is of particular interest for cooling apparatus for medical applications such as MRI systems. Accordingly, there is an ongoing desire to improve the performance of such PTRs, especially in relation to acoustic noise. When in use, PTRs generate a “chirping” noise which not only is a nuisance for operators of the equipment, but this noise can also translate into vibration, impacting sensitive measuring equipment and thus adversely affecting experimental results. This is a particular problem when PTRs are used for MRI imaging applications where, in addition to improving image resolution, there is a desire to reduce vibrations and audible noise since it is well established that many medical MRI procedures are aborted by patients when they become distressed during such procedures.
This problem has been addressed in GB-A-2391926, where the noise has been attributed to gas flowing at high speed over corrugations in the low pressure line between the valve assembly and the compressor. In order to reduce noise, the system of GB-A-2391926 comprises a dead end volume in fluid communication with the low pressure line, such that gas in the low pressure line is diverted into said dead end volume, therefore reducing the average gas velocity over the corrugations in the low pressure line. In a different embodiment, the diameter of the low pressure line is increased. However, the use of a bulky dead end volume close to the PTR is not desirable, and there is a continued desire to reduce the noise associated with a PTR.